Receivers may be used to estimate the location of a transmitter using a signal transmitted by the transmitter. As an example, receivers mounted on aircraft may be used to estimate the location of a transmitter on the ground. Known techniques may use differences between the signals as received by different receivers to estimate the location. According to a time difference of arrival technique, the difference in arrival times of the signal may be used to estimate the location. According to a frequency difference of arrival technique, the difference in the frequencies of the signal may be used to estimate the location. Known techniques, however, are not satisfactory in certain situations.